1. Field of the Present Invention
The present invention relates to a welding apparatus and a welding method for a nuclear fuel skeleton which is fabricated by welding guide tubes for power control rods and an instrumentation tube for measuring fuel to spacer grids having a plurality of cells, into which fuel rods are inserted. In particular, the welding apparatus and the welding method allow a robot having a welding gun to automatically spot-weld the guide tubes and the instrumentation tubes to the spacer grids.
2. Background of the Related Art
A nuclear fuel skeleton is a support structure for receiving fuel rods inserted into a reactor of a nuclear power plant as well as allowing inspection to reaction status of nuclear fuel. The skeleton includes spacer grids having a plurality of cells for the fuel rods and arranged at a certain span, guide tubes for control rods to be inserted into insert holes in the spacer grids and an instrumentation tube for measuring the status inside a reactor (hereinbelow, referred to as “an intra-reactor status”.)
The fuel rods are inserted into spacer grid cells of this skeleton and top and bottom nozzle are coupled to both ends of the skeletons to constitute a nuclear fuel assembly. Although the number of final fuel articles is varied according to unique models of nuclear power plants, about 177 fuel assemblies of this kind are inserted into a nuclear reactor and used for about 4 years on the basis of Korean Standard Nuclear Power Plant (KSNP) fuel.
The nuclear power plant requires high stability as reminded by the accident of Chernobyl nuclear power plant of the former Soviet Union in 1986. In particular, the nuclear fuel skeletons require super high stability so that they can be directly inserted into the reactor and endure extreme conditions for a long time. It is critical for a skeleton fabrication process to obtain a precise article through a number of inspections in a number of process steps.
The spacer grids and the guide and instrumentation tubes of the skeleton are coupled via bulging or welding.
TIG welding has been used as a conventional welding method for coupling. Since the spacer grids and the guide and instrumentation tubes are made of zircaloy, TIG welding is manually performed in a closed chamber filled with Ar gas owing to properties of zircaloy. If TIG welding is performed in the air, zircaloy metal in a welding zone will combine with oxygen owing to its properties to potentially break the welding zone, thereby causing welding to be impossible.
Another reason of manual welding is due to narrow spaces between the spacer grids and the guide and instrumentation tubes. This causes it to be difficult to obtain any welding angle as well as many risks that inter-electrode contact may occur in other skeleton regions during welding.
Further, automation of welding is difficult since every grid undergoes at least 40 points of welding in each tube.
Welding between the spacer grids and the guide and instrumentation tubes are still manually performed since a skeleton with the slightest defect is not reusable and such defects cause serious economical loss.
According to the above reasons, it has been required to automate the fabrication process of skeletons with poor productivity and high cost.